According to Victor Malo of Diario Gol (h/t the Mirror's Martin Domin), Guardiola believes an attempt to sign Messi, who will be available for free next summer as things stand, could jeopardise any chances he has of returning to the club one day.
Barcelona announced in the summer Messi would be remaining at the Camp Nou for four more years, but as the Guardian's Sid Lowe reported, that does not appear to be true:
Sid Lowe @sidlowe
Said Messi had *signed* new contract. Twice. He hasn't. In a year, he is a free agent.2017-9-2 08:48:22
As such, clubs can sign a pre-contract with the Argentina international in January ahead of a free transfer in the summer if that remains the case.
Guardiola spent the majority of his playing career at Barcelona and managed Messi at the club for four seasons, with the pair winning three La Liga championships and two UEFA Champions League titles together, among other trophies.
Now at Manchester City, Guardiola has assembled a fearsome attack that has racked up 40 goals in 12 Premier League games this season, and his project would likely be of interest to Messi if he were to leave Barca.
The 30-year-old, who has scored 523 goals for Barcelona in 602 appearances, is one of relatively few players who could enhance City's forward line.
ESPN FC's Michael Cox and NBC's Roger Bennett showered him with praise after he scored a brace against Juventus earlier in the campaign:
Michael Cox @Zonal_Marking
That goal, more than his 'blockbusters', is why Messi is so incredible. In one sense nothing special, in another completely unstoppable2017-9-12 19:36:24
roger bennett @rogbennett
Messi. Less a footballer. More a human wonder. Never for one second should we take for granted our ability to watch him play in the flesh2017-9-12 20:14:08
Barcelona cannot afford to lose him, and a move for him in January—when there still remains a possibility he could stay—could sour Guardiola's relationship with the Camp Nou faithful.
However, while Barca fans would be devastated if Messi left in any circumstances, it would seem unlikely they would hold Guardiola responsible for making an offer if the player had already left at the end of the season without having put an agreement in place in January.